1. Field of the Invention
This invention relates to gypsum-based construction materials. More particularly, the invention relates to a fungus resistant gypsum-based substrate faced with a synthetic polymeric sheet material that is suited for use as a construction material such as wallboard or ceiling panels. The invention is also directed to a process for manufacturing a fungus resistant gypsum-based substrate faced with a synthetic polymeric sheet material.
2. Description of Related Art
Fungi frequently can be found in the walls of buildings. Common fungi include mold and mildew. Fungi are especially troublesome in walls with poor ventilation where moisture can become trapped in the wall. The walls of portable buildings, such as temporary classrooms, have proved particularly susceptible to fungus growth because water often seeps in around the openings and joints of such structures. In buildings with poor ventilation or inefficient heating and air conditioning systems, the building walls are more likely to become breeding grounds for fungus. Some funguses that grow in walls, such as the stachybotrys chartarum (atra) fungus, produce toxins that have been known to render structures uninhabitable.
Conventional gypsum-based construction materials have the disadvantage that they support fungus growth when used in moist environments. Fungus needs both moisture and nutrients to survive. Naturally occurring organic matter that is a part of conventional gypsum board products, such as cellulose, paper fibers, starch, and contaminants, serve as nourishment for many strains of fungus. Accordingly, when conventional gypsum board becomes chronically moist or water damaged due to excessive humidity, water leaks, condensation, or flooding, fungus will grow on or in the gypsum board. Fungus growth can be exacerbated in gypsum board walls when vinyl wall coverings are used on the interior surface of the walls. Such vinyl wall coverings can trap moisture inside the gypsum board where it facilitates fungus growth.
Gypsum wallboard and gypsum panels are traditionally manufactured by a continuous process. In this process, a gypsum slurry is first generated in a mechanical mixer by mixing calcium sulphate hemihydrate (also known as calcined gypsum), water, and other agents. The gypsum slurry is normally deposited on a paper sheet. The gypsum slurry may include additives such as cellulose fibers that help to strengthen the gypsum core once it is dry. Starch is conventionally added to the gypsum slurry in order to improve the adhesion between the gypsum core and the paper facing. An upper continuously advancing paper sheet is laid over the gypsum and the edges of the upper and lower paper sheets are pasted to each other with a starch paste. The paper sheets and gypsum slurry are passed between parallel upper and lower forming plates or rolls in order to generate an integrated and continuous flat strip of unset gypsum sandwiched between the paper sheets that are known as facing or liners. This strip is conveyed over a series of continuous moving belts and rollers for a period of 2 to 5 minutes during which time the core begins to hydrate back to gypsum and hardens. During each transfer between belts and/or rolls, the strip is stressed in a way that can cause the paper facing to delaminate from the gypsum core if the adhesion between the gypsum core and the facing is not sufficient. Once the gypsum core has set sufficiently, the continuous strip is cut into shorter lengths or even individual boards or panels of prescribed length. Once again, it is important for there to be good adhesion between the paper sheets and the set, but still wet, gypsum core or the cutting action will pull the edges of the paper facing sheet away from the gypsum core.
After the cutting step, the gypsum boards are separated and grouped through a series of belts and rollers and then flipped over before being fed into drying ovens or kilns where the boards are dried so as to evaporate excess water. The hydration from hemihydrate to gypsum must be essentially complete at this point, normally between 7 and 15 minutes after mixing. When the gypsum boards are accelerated, flipped and fed into the drying ovens, the boards are subjected to a variety of stresses that can cause the facing to peel away from the gypsum core of the boards unless there is good adhesion between the set (but still wet) gypsum core and the facing material. Inside the drying ovens, the boards are blown with hot drying air at speeds up to 4000 feet/minute which can cause further delamination of the paper facing if there is not good wet adhesion between the gypsum and the paper liners. When portions of the facing sheets delaminate from the gypsum core during drying in the oven, the liner becomes entangled in the rollers and the gypsum crunbles as it dries which jams the oven and requires frequent shut downs of the line while the loose gypsum is cleaned out of the ovens. The gypsum boards are dried in the ovens for anywhere from 30 to 75 minutes. After the dried gypsum boards are removed from the ovens, the ends of the boards are trimmed off and the boards are cut to desired sizes.
The fully dried gypsum adheres well to the paper facing sheet materials as long as the gypsum board is kept dry. However, paper facing has a number of inherent properties that can be detrimental in a gypsum wallboard product. As discussed above, paper facing material (sometimes called a paper liner) is made of cellulose which serves as a nutrient for fungus growth. Paper facing also is not as strong or abrasion resistant as needed for certain construction applications. In addition, because the strength of paper differs significantly depending on the direction in which the strength is measured, paper facing must be relatively thick in order to achieve satisfactory multidirectional strength. Paper faced gypsum-board products also suffer from a lack of abrasion resistance. Paper facing used on conventional gypsum board becomes especially weak and subject to delamination from the gypsum core when the paper becomes damp due to leaks or high humidity.
Paper-faced gypsum boards must generally be coated with another material, such as paint or a wallcovering material, in order to achieve sufficient abrasion resistance. For example, paper-faced wallboard is often covered with vinyl wallcovering, a hard plastic sheet, or a plastic film when used in high traffic areas. Unfortunately, such coatings and coverings tend to trap moisture inside the wall where it can precipitate fungus growth.
Canadian Patent No. 1,189,434 discloses gypsum panels made with a facing of a moisture vapor permeable spunbonded nonwoven material. Canadian Patent No. 1,189,434 discloses gypsum panels faced with Tyvek(copyright) spunbonded olefin sheet material. Tyvek(copyright) is a registered trademark of E.I. du Pont de Nemours and Company of Wilmington, Del. Tyvek(copyright) sheets are made by solution flash-spinning polyethylene to form fine plexifilamentary fibril structures that can be thermally bonded to form sheet material. U.S. Pat. No. 5,704,179 discloses gypsum board faced with mats of fiberglass or synthetic resin fibers. While the panels disclosed in these patents eliminate naturally occurring organic matter from the facing sheets of the gypsum board, these patents are not directed to reducing or eliminating fungus growth. Accordingly, the patents do not disclose removal of nutrients from the gypsum core or other enhancements needed to reduce fungus growth in the gypsum board.
In addition, while it has been possible to produce gypsum boards faced with polymeric fibrous sheet materials on a small laboratory scale, it has proven difficult to produce gypsum boards faced with such sheets on a commercial scale. This is because the adhesive strength between conventional fibrous synthetic fibrous sheets and the wet gypsum core (known as wet adhesion) tends to be low. Thus, the facing peels away from the gypsum core during various points in the production process before the boards are fully dried in the drying ovens.
There is a need for a process by which gypsum board that is free of fungus nutrients such as cellulose, starch, and natural fibers that can be manufactured on a commercial basis. There is also a need for a gypsum board that does not trap mold supporting moisture. Finally, there is a need for gypsum boards that actually include substances that prevent the growth of fungus.
The present invention is directed to a fungus resistant gypsum board, comprising: a first polymeric fibrous sheet, the first sheet having a first surface and opposite first and second edges; a second polymeric fibrous sheet, the second sheet having a first surface and opposite first and second edges; a gypsum core sandwiched between the first and second nonwoven sheets, the gypsum core containing less than 0.03% by weight, based on the weight of the dry gypsum core, of formulation additives that serve as fungus nutrients; and a synthetic adhesive on the first and second edges of said second sheets, the synthetic adhesive adhering the first edge of said first nonwoven sheet to the first edge of the second nonwoven sheet, and adhering the second edge of the first nonwoven sheet to the second edge of the second nonwoven sheet. Preferably, the gypsum core contains less than 0.5% by weight, based on the weight of the dry gypsum core, of fungus nutrients.
According to a preferred embodiment of the invention, the gypsum core contains a fungicide such as a metal/inorganic derivative. More preferably, the fungicide is boric acid, and the gypsum core is comprised of between 0.04 and 0.25 weight percent, based on the weight of the dry gypsum core, of boric acid.
According to a preferred embodiment of the invention, the first and second polymeric fibrous sheets are nonwoven sheet. In the preferred embodiment of the invention, the first surface of the first nonwoven sheet and the first surface of the second nonwoven sheet adhere to said wet gypsum core with an adhesive strength of at least 7.5 lb. The first surface of the first nonwoven sheet and the first surface of the second nonwoven sheet have pores containing set gypsum of the gypsum core intertwined with the fibers in the first and second nonwoven sheets. The sheets may be comprised of a needle punched staple fiber sheet, a hydroentangled fibrous sheet, or a spunbond sheet. Alternatively, the first surface of the first and second nonwoven sheets may be coated with a primer layer of a high density gypsum slurry having a density that is at least 1.1 times the density of the gypsum core. Preferably, the first and second sheets adhere to said wet gypsum core with an adhesive strength of at least 10 lb.
The present invention is also directed to a process for manufacturing a gypsum-based substrate. The process includes the steps of: adding calcined gypsum, formulation additives and water to a mixer, the mixture containing less than 0.02% by weight, based on the weight of the total slurry mix , of formulation additives that serve as fungus nutrients; mixing the gypsum and water in the mixer to produce a gypsum slurry that is comprised of 50 to 65 weight percent gypsum; providing a first polymeric fibrous sheet, the first sheet having a first surface and opposite first and second edges; pouring the gypsum slurry from the mixer onto the first surface of the first sheet and spreading the gypsum slurry over the first surface of the first sheet; providing a second polymeric fibrous sheet, the second sheet having a first surface and opposite first and second edges; applying a synthetic adhesive on the first and second edges of the second sheet; placing the first surface of the second sheet over the gypsum slurry that has been spread over the first surface of the first sheet; adhering the adhesive on the first edge of the second sheet to the first edge of the first sheet, and adhering the adhesive on the second edge of the second sheet to the second edge of the first sheet; enclosing the gypsum slurry between the first and second sheets to bring the slurry into intimate contact with the first and second sheets and form an elongated strip of gypsum slurry sandwiched between the first and second sheets; allowing the elongated strip of gypsum slurry to set up and harden to form a stiff elongated strip having a solid, wet gypsum core sandwiched between the first and second sheets; cutting the stiff elongated strip into gypsum board of desired length; drying the gypsum board in a dryer to remove excess water from the gypsum boards.
In the process of the invention, the gypsum slurry contains less than 0.33% by weight, based on the weight of the total slurry, of fungus nutrients. After the elongated strip of gypsum slurry has set up and hardened to form a stiff elongated strip having a solid, wet gypsum core sandwiched between the first and second sheets, the first and second sheets preferably adhere to the wet gypsum core with and adhesive strength of at least 7.5 lb. According to the more preferred process of the invention, the first and second sheets are nonwoven sheets. It is preferred that the first surface of the first nonwoven sheet and the first surface of the second nonwoven sheet have open pores between fibers of sufficient size for the gypsum slurry to enter the pores and become intertwined with the fibers in the sheets when the gypsum slurry is enclosed between the first and second nonwoven sheets. It is desirable that the first and second sheets each have a mean flow pore size, measured according to ASTM F316-86, of at least 8.0 microns, and more preferably in the range of 8.7 to 40 microns.